i have this exact problem! i changed my fuel pump and still didn't work! i have a feeling could be my injectors!!?
have you had it looked at, i seen you wrote above you might take it too alstar what did they say, rather fix myself if its nothing huge??
otherwise i will book it in a few weeks?? and let you know what they say, if you haven't taken it in yet!

Might be boost cut.

Sounds more like AFM issues or coilpacks/plugs/leads.

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03 STi @ 280hp/1.3bar